1. Field of the Invention
The present invention relates to filtration and retention screens positioned in fluid pathways. More particularly, the present invention relates to means for removing and re-inserting such screens from or into the fluid pathway without halting the process generating, treating or using the fluid. The present invention relates to retractable material retaining screens.
2. Description of the Prior Art
Effective fluid flow transfer is an important aspect of many industrial processes. In the power generation industry in particular, the effective transfer of significant volumes of fluids impacts power generation productivity and the environment. Devices designed to ensure that such fluids move from one portion of the power generation plant to another when desired aid in maximizing productivity and minimizing adverse environmental impact. However, as power generation facilities and systems increase in size, the task of fluid diversion devices becomes increasingly harder.
It is well known in the power generation industry that boilers are employed to produce steam at high temperature and pressure. That steam is used to move turbines coupled to generators. Combustible fuels such as coal, oil, gas, etc., are used to produce the heat necessary to generate the steam. Products of that fuel combustion exit the boiler at high temperatures and can include a variety of byproducts, including particulate matter, dependent upon the type of fuel. The high-temperature combustion products exiting the boiler may be exhausted to the atmosphere through a stack, transferred to various filtration and scrubbing systems, or both in alternation under a schedule or as conditions warrant. In some instances, Selective Catalytic Reduction (SCR) reactors are used with fossil fuel-fired boilers to reduce nitrogen oxide (NOx) emissions generated in the combustion process. These reactors are normally installed downstream of the boiler, upstream of air preheaters.
Fly ash, one form of the particulate matter generated in the combustion process, may be a substantial and undesirable byproduct. This particulate matter is generally transferred with the combustion gas through one or more components of the power generation system but efforts are made to remove the fly ash to the greatest extent possible in order to maximize process efficiency and to meet governmental emission requirements. Filter devices are employed for that purpose. In the context of the present invention, a filter is to be understood to include any mechanism configured to allow a fluid to pass there through while blocking the passage of particulate matter. The size of the particulate matter blocked is dependent upon the size of the openings of the filter mechanism and the angle of impingement. A filter may be a filter as generally understood, or a screen, either of which may be a metallic or nonmetallic mesh, weave, knit, spun material, or fabric.
In addition to filters used to filter out particulate from the fluid pathway, the SCR reactors are used to remove certain gases from the boiler flue gas. SCR reactors include one or more layers of catalyst beds to facilitate removal of the NOx emissions. During NOx removal, a reagent, such as ammonia gas or a suitable equivalent, is injected with the flue gas into the SCR reactor vessel. Two types of catalyst beds of defined geometry are generally used in the SCR reactor. The two types typically used are: 1) honeycomb-type (or grid-type) and 2) plate-type. Either of the two catalyst beds is normally assembled into standard commercial-size modules to facilitate loading and handling in approximately half-meter or one-meter increments per layer. The catalyst is suspended within the SCR reactor, ordinarily in a plurality of layers, with the catalyst installed one-half to one-meter in depth per layer.
In an exemplar processing operation, the flue gas enters the SCR's first catalyst layer at a velocity of about 8–20 feet per second. The flue gas passes through holes (honeycomb-type) or slots (plate-type) in the first catalyst layer, exits the first catalyst layer, enters the second catalyst layer, and so on. Holes or slots (also known as hydraulic diameter or pitch opening) in the catalyst layer are normally about 3 mm to 8 mm, closely spaced. In this manner, 70% to 95% of the catalyst layer surface is open to passage of flue gas through it.
Fly ash particle size distribution and particle sizes are highly dependent on the nature of fuel burned and boiler process conditions. In general however, fly ash particles entering the SCR reactor can range in size from about 0.01 mm to about 3 mm in diameter. However, these particles do agglomerate with each other causing particle sizes of 1 cm or larger to form what is often referred to as popcorn ash. Particles larger in size than the available catalyst pitch opening, cannot traverse through the catalyst layer, hence these particles collect and continue to build up upstream of the catalyst layer. Moreover, particles nearly equal in size to catalyst hydraulic diameter often lodge inside the catalyst in the holes or slots. Filters upstream of the SCR reactor reduce particulate accumulation in the catalyst beds, but such accumulation does occur in the filters. While filtration can be achieved by capture of the particles within the filter, a more effective method involves impingement of the particles against a screen and subsequent deflection of the particles into an appropriate receiver.
The agglomeration of particles, whether at the filter, the SCR catalyst beds, or both, can have a significant adverse impact on the efficiency of the SCR reactor and/or the power generation system. Specifically, the effective fluid pass through area is reduced, increasing pressure drops throughout the system. In the SCR reactor, particulate accumulation reduces the reaction zone of the SCR reactor and so the reaction time is diminished. This naturally affects the entire energy generation system in an adverse way and so it is undesirable to have a build up of particles in the SCR reactor in particular, and throughout the system fluid pathway in general. Since power generation systems, particularly those including SCR reactors, are designed in fine balance, it is important that all subsystems operate substantially as designed. When the operating conditions change, the balance of the entire reaction process and therefore the power generation can be altered adversely. In sum, there is a fine balance in the system and plugging of the fluid pathway, the SCR reactor, or both, throws that balance off.
During the filtering/screening process, certain areas of the filter will be plugged by the particulate. This plugging will result in increased pressure losses through the filtering media and increased velocities in the remaining open areas. The increased pressure losses will result in reduced efficiencies in the system and higher operating costs. The increased velocity of the flow through the open area will result in elevated erosion rates in those open areas, resulting in eventual failure of the abraded region. Repair or replacement of the damaged regions currently require shutdown of the process.
Presently, there are two methods for removing particulate buildup on filters and for cleaning catalyst beds of SCR systems. The first involves particulate removal with the filter or catalyst bed in place. The second involves pulling the filter or catalyst bed out of the fluid pathway and replacing or cleaning it. Both options require shutting down the entire process, an option that is undesirable in the power generation industry, given the time and expense involved in shutting down and restarting a boiler. Therefore, what is needed is a system for removing particulate from filters or catalyst beds, or for refreshing or replacing the catalysts of SCR catalyst beds. Further, what is needed is such a system that may be done without requiring a shutdown of the fluid generation and movement process.